I'm assuming pretty much everyone has played the game by now so I will expand on the above answer:
The master Chief and the Arbiter have activated the new Halo ring and race to the frigate "Forward Unto Dawn" in a UNSC Warthog. They make it just in time and the Chief narrowly escapes being sucked into the gravity of the collapse of the premature firing of the replacement for installation 04 as the Arbiter takes command at the bridge (I assume Cortana has full control). Cortana says to the Chief "It's been a pleasure serving with you, John."
We next see the Dawn crash land in a body of water and the Arbiter emerging with shocked looks from some human rescuers. Lord Hood and the remaining survivors back on earth are holding a remembrance service and Lord Hood says to the Arbiter, "I remember how this war started, what your kind did to mine. I cannot forgive you. But you have my thanks for standing by him to the end. Hard to believe he's dead." They shake hands and the Arbiter replies with, "Were it so easy." The Arbiter is then seen on his own ship and sets off with his remaining Elites back to their home planet.
The credits roll and after they end we see the Master Chief still floating in a non gravity atmosphere within the Dawn. Cortana explains that when the ring was activated prematurely it "Did a number on the Ark." This caused the portal to become unstable and only the front most part of the frigate made it through leaving the Master Chief and Cortana floating in space in the remains. Cortana lays an emergency beacon and comments that it will be months before they are found, years even. The master Chief enters a cryo tube and prepares to activate it. Cortana states, "I'll miss you." The Master Chief replies with, "Wake me, when you need me."
However in the Halo 4 Master Chief was probably dead because they show inside of him. Cortana yells at Chief to wake up. When he woke up, Chief suddenly burst out and that the ship he was on was blowing up. He got Cortana and use his jet pack (We didn't know that he had a jet pack all the time and didn't use it). When he got out he and Cortana are heading to an unknown world. The place that threatened the universe. So it isn't the end of Halo. Halo 4 comes out this 2012. 343 Industries takes over for Bungie